Polycyclic aromatic hydrocarbons (PAHs) are supposed to be the carriers of
the diffuse interstellar bands (DIB). Most favoured theories concerning the
DIB-phenomenon propose that the observed radiation originates from infrared
fluorescences of vibronic highly excited PAHs. Photodissociation of PAHs
has been shown to be the principal process competing with radiative
relaxation of these PAHs. Due to their high resilience towards
decomposition PAHs are attractive compounds for photoionization
studies.

In the present study light from the Berlin synchrotron BESSY was used to
investigate the fragmentation of various PAHs by means of standard
ionisation and coincidence techniques.

After having been ionized using photon energies between 10 eV and 30 eV
various substances have been analyzed in a McLaren-type time of flight mass
spectrometer.

At sufficiently high photon energies the studied PAHs dissociate by
splitting off H-atoms and CxHy-fragments. The critical energies are
determined and compared with appropriate values from electron bombardement
ionisation.
By means of experimentally taken breakdown curves the isomerisation
processes of the studied substances are discussed. At least the breakdown
curves mentioned before are compared with RRKM calculations.